MONDAY, Nov. 12 (HealthDay News) -- Only one-quarter of smoking parents adopt a strict smoke-free car policy, and nearly half who don't enforce such a ban light up while driving with their children, a new study indicates.

Interviewing nearly 800 smoking parents, researchers also found that two out of three parents with strict smoke-free home policies don't match that stance in their cars. Nearly three-quarters of smoking parents admitted that someone had smoked in their car in the last three months -- suggesting parents don't recognize the dangers of exposing their kids to tobacco residue in such a confined space.

"We've seen that a high number of parents don't smoke in their homes and expected the same kind of [behavior] in cars, so we were shocked and surprised," said study author Dr. Emara Nabi-Burza, a senior clinical research coordinator at the Center for Child and Adolescent Health Research and Policy at Massachusetts General Hospital for Children in Boston.

"For some reason, the car isn't considered an environment where children can be exposed to tobacco smoke," she added. "Parents think putting down the windows is fine. They don't think of it as an indoor exposure for children, which is where we need to step in and make people aware."

The study is published online Nov. 12 in the journal Pediatrics in advance of publication in the December print issue.

No safe level of tobacco smoke exposure exists, according to the U.S. Surgeon General, and research has shown that it contributes to a worsening of asthma symptoms in children and greater odds of respiratory infections, sudden infant death syndrome and ear infections. In children aged 18 months or younger, exposure to so-called secondhand smoke is responsible for up to 15,000 hospitalizations in the United States each year, the study said.

Nabi-Burza and her colleagues, interviewing parent smokers as they exited pediatricians' offices in eight states, learned that 48 percent of those without a strictly enforced smoke-free car policy smoked while driving with their children. College-educated parents of children under 1 year were more likely to enforce such a policy, as were those who smoked 10 or fewer cigarettes per day.

Only 12 percent said they had been advised by their children's doctors to have a smoke-free car.

"Mostly we see when pediatricians talk to parents, it's about smoke-free homes," Nabi-Burza said. "Even bars are smoke-free, but cars have been kind of forgotten. Now that we know the extent of the problem, pediatricians should talk to parents about how smoking in cars is not good for children."

Danny McGoldrick, vice president of research at the Campaign for Tobacco-Free Kids in Washington, D.C., noted that even tobacco smoke residue -- so-called "thirdhand smoke" -- in cars can be harmful to children, increasing the importance of smoke-free car policies even if youngsters aren't present while a parent smokes.

"Fabrics obviously absorb a lot of these toxic components. Just because no one's in there smoking doesn't mean all the harmful [components] disappear," McGoldrick said. "The best thing to do as a smoking parent is to quit smoking. If they're not ready to quit yet or not able to succeed, then adopt smoke-free policies for your home and car."